[Bug tree-optimization/91996] New: fold non-constant strlen relational expressions

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91996

            Bug ID: 91996
           Summary: fold non-constant strlen relational expressions
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Now that the strlen pass is integrated with EVRP it's become possible to make
use of the EVRP ranges to fold even some relational expressions involving
non-constant strlen values.  For example, in the test case below, since a's
length is known to be at least 7, copies of its first N characters must be at
least as long as MIN (N, strlen (a)].

$ cat b.c && gcc -S -O2 -Wall -fdump-tree-optimized=/dev/stdout b.c
extern char a[32], b[32];

void f (void)
{
  if (__builtin_strlen (a) < 7)
    return;

  __builtin_memcpy (b, a, sizeof b);

  if (__builtin_strlen (b) < 7)   // can be folded to false
    __builtin_abort ();
}

;; Function f (f, funcdef_no=0, decl_uid=1926, cgraph_uid=1, symbol_order=0)

Removing basic block 6
Removing basic block 7
f ()
{
  long unsigned int _1;
  long unsigned int _2;

  <bb 2> [local count: 1073741824]:
  _1 = __builtin_strlen (&a);
  if (_1 <= 6)
    goto <bb 5>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 3> [local count: 708669605]:
  MEM <unsigned char[32]> [(char * {ref-all})&b] = MEM <unsigned char[32]>
[(char * {ref-all})&a];
  _2 = __builtin_strlen (&b);
  if (_2 <= 6)
    goto <bb 4>; [0.00%]
  else
    goto <bb 5>; [100.00%]

  <bb 4> [count: 0]:
  __builtin_abort ();

  <bb 5> [local count: 1073741829]:
  return;

}